Sourav Banerjee Speaking about Ultrasound treatments on bast fibres like hemp, it increases specific surface by leading to fibrillation in retting stages. These fibrillation process will lead to higher surface area and thereby will lead to larger diffusion of dye molecules into the fibre.

Ultrasonic treatment for scoured wool in (i)water and (ii)surfactant media will have different effects. Ultrasonic treatment generally leads to cavitation effect which leads to a) breaking up of aggregates of dye molecules b)removing the entrapped air in the interstices of fibres c)disrupting the fibre liquid boundary d)increasing the swelling of fibre. Whether ultrasonication leads to similar effects of fibre modification in protein fibres is an area of probe.

I agree with Anupam. I prefer to use ultrasonic bath, my group used ultrasound instead of reflux , the result is much better at shorter time. The solution becomes homogeneous and produce uniform dying .

As has already been said, ultrasound leads to a more intense and more homogeneous dyeing result on wool and other fibers. The diffusion of dyestuffs into the fibers will be enhanced by ultrasound. The action of ultrasound is on the one hand a homogenization of the dyebath and improvement of the solubilization of the dyestuffs in the bath and on the other hand an improvement of the dyestuff uptake on and in the fiber.

We studied the action of ultrasound during wool dyeing some years ago. I have attached three papers on this subject. Maybe they are helpful for you.